Transducers of the above type are generally described in an article entitled "New Developments in Force Measurement" by Joseph H. Antkowiak et al., published by Hottinger Baldwin Measurements, Inc., Marlboro, Mass., 01752. This type of transducer, due to its structure and function, has many advantages, for example, it is highly accurate, structurally it has a relatively low profile and permits practical load introduction techniques, while not requiring large moment supporting foundations. However, there is room for improvement, especially with regard to eliminating undesirable mounting effects, and effects caused by off-center load applications, and improved manufacturing costs.
U.S. Pat. No. 4,811,610 (Gassmann) describes a weighing cell quite similar to the transducer or load cell described in the above mentioned article. Gassmann wants to improve such transducers, especially with regard to their sensitivity to off-center load applications, and to mounting effects. In other words, Gassmann wants to make his load cells less sensitive to these phenomena. This aim is presumably achieved in the Gassman transducer by an especially small web thickness (t) relative to the axial heigh (h) of the sensing ring or deformation body which is connected by the webs on the radially inner side, to the force application body, and on the radially outer side to the ring housing of the transducer, and by the requirement of a narrow deep radial groove width (b) between the sensing ring and the ring housing. The groove width (b) is considered relative to the radial width (l) of the deformation body or sensing ring. The groove (b) must be deep to satisfy the requirement that the web thickness (t) is not more than one sixth of the height (h) of the sensing ring and the groove width (b) is not more than one third of the radial sensing ring width (l).
These limitations imposed by Gassmann are undesirable because even modern machine tools have difficulties in cutting such deep and narrow grooves due to required cutting tool dimensions, which normally do not permit boring such narrow deep axial grooves without special precautions and at economical cutting speeds.
Although Gassmann intended to virtually eliminate the effects of disturbing variables occurring during load introduction and in the mounting of the weighing cell, achieving such goals has been difficult in practice due to the limitations imposed by trying to solve the problem exclusively at the webs which mount the sensing ring to the housing. The invention is based on the recognition that modifications in the web and groove dimensions are limited in their isolating abilities due to the above mentioned machining problems.
Mounting effects or "variables" are influences that affect the measuring result so that consistently repeatable results are hard to obtain, if the same transducer model is mounted to different types of foundations or even if in the same type of mounting the several mounting screws are not tightened to the same extent, or if the mounting surfaces have different surface characteristics, e.g., different roughnesses and flatness.